Let us think a little bit about planet nine and the observation there of, a few back of the envelope observations…
Neptune, which should have a radius about twice as large, had a visual magnitude of about 8 (invisible to the naked eye, but can be found with a small telescope).
Planet nine which should theoretically be about half as large would reflect 1/4th the light of Neptune, if it were at the same distance, unfortunately it is ten times as far out.
It is all of those 10’s that make this a difficult hunt…
10 times as far out means that the amount of light reflected is going to be 1/100th as much….
But 10 times as far out also means that it will be getting 1/100th as much light from the sun…
This means that the amount of light coming from Planet Nine would be 1/4th times 1/100th times 1/100th as dim as Neptune…
or 1/40,000th as bright.
Starting with the magnitude 8 of Neptune we would end up with a apparent magnitude around 19.5!
An object with that kind of a magnitude would not be impossible to resolve from Earth, but extremely difficult.
Now we have the next problem, at that magnitude there is a whole lot of stuff no matter what direction in the sky you look in, the name planet comes from “wanderer” because the planets wander across the “fixed” stars in the sky, but the orbital motion across the sky of an object at this distance and with an orbit of 10,000+ years would mean that the motion across the fixed sky over a short observation time would be very, very small.
Once again, not impossible to find, but still, very small.
The next caveat is of course we have no idea what the surface is like. If it is ice and gets beat up on a geologically frequent basis then we have a possibility that it reflects light in a reasonable fashion. Of course, it may also be a soup of frozen hydrocarbons and be as black as coal.
It has been seen previously that some of the objects floating out there past Neptune are not the whitest objects in the solar system, if we are lucky though it is possible that an object of this mass wold be able to maintain an atmosphere, and if we are really lucky that atmosphere would contain clouds of a gas that has some reflective properties…
The caveats keep stacking up though.
What we need are some more clues as to where to look, because the best way to find a needle in a haystack is to first burn the haystack.
This planet is being proposed because it has an ability to perturb the orbits of other small (planetoid) objects in the Kuiper belt.
The Kuiper belt is home to more that just planetoid sized objects, there are also occasional reasonably sized ice balls chucked towards the sun from this range affectionately known as comets.
A lot of the comets that we see are old fogies that have been kicking around the inner solar system from some time and have been getting tossed around by the gravities of the inner planets (I’m look at you Jupiter). Occasionally we get an oddball though, like Catalina that is currently streaming on it was out after a one way trip through the inner solar system.
If there is a large object floating out at the edge of the solar system it could be effecting the orbits of objects and directing them off into new orbits, if you could find a pattern of where these perturbations took place, like if the orbits of “one and done” comets seem to fall along a track across the sky perhaps you can get some more clues as to the movement of a massive object out in the area where Planet Nine is hypothesized to be floating.
If someone was kind enough o offer me an opportunity to attend a grad school I would learn the orbital mechanics and start figuring out the tracks of the long list of observed “one and done” comets that have streamed through the inner solar system since we have been making reliable observations.
OK, I am not done yet, I had some horrific allergies earlier and as a result took a long afternoon nap and I am no where near tired yet.
The part of the “Solar system” where Planet Nine would be orbiting would be on the backside of the Heliopause. This means that it would be kick around out in the interstellar medium, and not in the portion of the solar system that is supplied with volatile compounds from the sun, and the solar wind blowing materials off of the other planets. This could mean two things. The first of which is that the region in which it is inhabited contains a different mix of compounds that would be raining down into its atmosphere, and likely due to the low temperatures, actually able to stay there. Things like Hydrogen and Helium would likely make up the majority of the atmosphere while the lower part would be a soup of ammonias, ices (likely some of the odd forms of ice that are only stable under low temperatures and higher pressures, methane, and the like. The other side of this card is that since it is kicking around outside of the heliopause it would not have the protection of the sun when it comes to solar wind bursts coming from other stars, this would result in its atmosphere having a greater possibility of being stripped on occasion.
Of course it is also possible that when this stripping occurs much of the material is not blow all that far away and still remains within what would likely be a considerable gravity well formed by this object, especially considering that there is not a heck of a lot of other competition from materials by massive objects in that range. So if the planet is occasionally stripped of materials it could then form a nice ring system for a period of time, not sure on the stability of such a ring system, but if it did occur and it was aligned correctly (which, odds are, it wouldn’t be), it would make the planet a much better target.
Maybe some of the observations of Voyager in the coming years will give us some more clues about the region in which Planet Nine is inhabiting, and then we can make some more reasonable predictions, but here is my prediction, just pulling this out of Uranus… (sorry)
Rocky core covered with ices and mixed with a soup of hydrocarbons. This is covered over with a liquid layer composed largely of light hydrocarbons. The atmosphere would be largely hydrogen and helium, with clouds of methane as the low surface temperature would dictate that these would be most of what would be available.
Atmospheric composition should be closer to the Interstellar medium, with about 70% hydrogen, 28% helium and the rest methane and others (nitrogen). This would mean that if you were to observe it it would likely be a faint blue color. The blue coming from the portion of the methane that would be present and visible in the atmosphere, with the rest being largely transparent.
If we were lucky the lower surface, which unlike the gas giants, may still be visible with much of the atmosphere being tansparent, would be covered by ices that reflect whitish and not longer hydrocarbons where the lower surface would be like asphalt.
On a side note, I am going to just keep adding to this until it gets to the point where I have enough information to make another blog out of this. If I start trying to figure out the orbita parameters of comets that will get chucked into another blog.